Add stem rendering support for tablature notation

[brianshano]

Addressing feature request #948

An attempt to add note stems to the tab numbers.
In progress - working on stem alignment

[paulrosen]

Thanks for this!

Be sure to test this will multi-voice and multi-staff music. I think there are some vertical spacing problems that I haven't gotten around to addressing.

And tabs with different numbers of strings.

There are a number of tests already, and you can add more, or I can. See https://docs.abcjs.net/developers/basic-architecture.html#testing for more info.

[paulrosen]

Not sure about your changes to relative - that is going to affect non-tablature music, too, I think.